System and method for achieving continuous connectivity to an access point or gateway in a wireless network following an on-demand routing protocol, and to perform smooth handoff of mobile terminals between fixed terminals in the network

ABSTRACT

A system and method for enabling an ad-hoc communication network to maintain connectivity with the mobile nodes in the network in an effective and efficient manner with minimal overhead. The system and method enables an ad-hoc communication network to maintain connectivity between intelligent access points of the network and mobile nodes in the network while performing an on-demand protocol. The system and method further uses an improved distance vector routing algorithm and unicast messages, to thus avoid an increase routing advertisement frequency in the network while keeping network overhead at a minimum. The system and method also modifies the Ad Hoc On-Demand Distance Vector Routing (AODV) protocol to facilitate smooth handoff of subscriber devices in an ad-hoc communication network while also eliminating unidirectional links between nodes in the network.

The present invention claims benefit under 35 U.S.C. § 119(e) from U.S.Provisional Patent Application Ser. Nos. 60/439,448, 60/439,449 and60/439,455 of Avinash Joshi, each filed on Jan. 13, 2003, and from U.S.Provisional Patent Application Ser. No. 60/476,237 of Avinash Joshi,filed on Jun. 6, 2003, the entire contents of each being incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and method for enabling anad-hoc communication network to maintain connectivity within mobilenodes and fixed nodes in the network in an effective and efficientmanner with minimal overhead. More particularly, the present inventionrelates to a system and method for enabling an ad-hoc communicationnetwork to maintain connectivity between intelligent access points ofthe network and other nodes (e.g., wireless routers and subscriberdevices which can be fixed or mobile) in the network while using anon-demand routing protocol. The present invention also relates to asystem and method for achieving smooth handoffs of mobile nodes betweenfixed nodes in an ad-hoc communication network using an improveddistance vector routing algorithm and unicast messages. Furthermore, thepresent invention relates to a system and method for modifying the AdHoc On-Demand Distance Vector Routing (AODV) protocol to facilitatesmooth handoff of subscriber devices in an ad-hoc communication networkwhile also eliminating unidirectional links between nodes in thenetwork.

2. Description of the Related Art

Wireless communication networks, such as mobile wireless telephonenetworks, have become increasingly prevalent over the past decade. Thesewireless communications networks are commonly referred to as “cellularnetworks”, because the network infrastructure is arranged to divide theservice area into a plurality of regions called “cells”. A terrestrialcellular network includes a plurality of interconnected base stations,or base nodes, that are distributed geographically at designatedlocations throughout the service area. Each base node includes one ormore transceivers that are capable of transmitting and receivingelectromagnetic signals, such as radio frequency (RF) communicationssignals, to and from mobile user nodes, such as wireless telephones,located within the coverage area. The communications signals include,for example, voice data that has been modulated according to a desiredmodulation technique and transmitted as data packets. As can beappreciated by one skilled in the art, network nodes transmit andreceive data packet communications in a multiplexed format, such astime-division multiple access (TDMA) format, code-division multipleaccess (CDMA) format, or frequency-division multiple access (FDMA)format, which enables a single transceiver at the base node tocommunicate simultaneously with several mobile nodes in its coveragearea.

In recent years, a type of mobile communications network known as an“ad-hoc” network has been developed. In this type of network, eachmobile node is capable of operating as a base station or router for theother mobile nodes, thus eliminating the need for a fixed infrastructureof base stations. Details of an ad-hoc network are set forth in U.S.Pat. No. 5,943,322 to Mayor, the entire content of which is incorporatedherein by reference.

More sophisticated ad-hoc networks are also being developed which, inaddition to enabling mobile nodes to communicate with each other as in aconventional ad-hoc network, further enable the mobile nodes to access afixed network and thus communicate with other mobile nodes, such asthose on the public switched telephone network (PSTN), and on othernetworks such as the Internet. Details of these advanced types of ad-hocnetworks are described in U.S. patent application Ser. No. 09/897,790entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced tothe PSTN and Cellular Networks”, filed on Jun. 29, 2001, in U.S. patentapplication Ser. No. 09/815,157 entitled “Time Division Protocol for anAd-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access toShared Parallel Data Channels with Separate Reservation Channel”, filedon Mar. 22, 2001, and in U.S. patent application Ser. No. 09/815,164entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile RadioAccess System”, filed on Mar. 22, 2001, the entire content of each beingincorporated herein by reference.

As can be appreciated by one skilled in the art, since certain nodes ofthe ad-hoc network are mobile, it is necessary for the network tomaintain connectivity with those nodes. Accordingly, needs exist forimproved techniques to enable an ad-hoc network to maintain connectivitywith the mobile nodes in the network in an effective and efficientmanner with minimal overhead. Similarly, most of the traffic flowsthrough the Access Point (AP) in such a network and hence, there exist aneed for all nodes to maintain routes with the Access Point (AP) all thetime in an effective and efficient manner with minimal overhead.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a system and method forenabling an ad-hoc communication network to maintain connectivity withinthe mobile nodes and fixed nodes in the network in an effective andefficient manner with minimal overhead.

Another object of the present invention relates to a system and methodfor achieving smooth handoffs of mobile nodes between fixed nodes in anad-hoc communication network.

These and other objects are substantially achieved by providing a systemand method for enabling an ad-hoc communication network to maintainconnectivity between intelligent access points of the network and othernodes (mobile nodes and fixed nodes) in the network while using anon-demand routing protocol.

These and other objects are further substantially achieved by providinga system and method, for use in an ad-hoc communication network, whichenables the network to perform smooth handoffs of mobile nodes betweenfixed nodes in an ad-hoc communication network using an improveddistance vector routing algorithm and unicast messages, thus avoiding anincrease in routing advertisement frequency while keeping overhead at aminimum.

This and other objects are still further substantially achieved byproviding a system and method for modifying the Ad Hoc On-DemandDistance Vector Routing (AODV) protocol to facilitate smooth handoff ofsubscriber devices in an ad-hoc communication network while alsoeliminating unidirectional links between nodes in the network.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and novel features of the inventionwill be more readily appreciated from the following detailed descriptionwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of an example ad-hoc packet switched wirelesscommunications network including a plurality of nodes in accordance withan embodiment of the present invention;

FIG. 2 is a block diagram illustrating an example of a mobile nodeemployed in the network shown in FIG. 1;

FIG. 3 is a conceptual block diagram of an example of the relationshipbetween wireless routers and access points of the network shown in FIG.1;

FIG. 4 is a conceptual block diagram of an example of the relationshipbetween a moving subscriber device with respect to stationary wirelessrouters of the network shown in FIG. 1;

FIG. 5 is a conceptual block diagram of an example of the relationshipbetween a moving subscriber device with respect to stationary wirelessrouters and access points of the network shown in FIG. 1; and

FIG. 6 is another conceptual block diagram of an example of therelationship between a subscriber device, wireless routers and an accesspoint of the network shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram illustrating an example of an ad-hocpacket-switched wireless communications network 100 employing anembodiment of the present invention. Specifically, the network 100includes a plurality of mobile wireless user terminals 102-1 through102-n (referred to generally as nodes 102, mobile nodes 102 orsubscriber devices (SD)), and can, but is not required to, include afixed network 104 having a plurality of intelligent access points 106-1,106-2, . . . 106-n (referred to generally as nodes 106, access points106 or IAPs), for providing nodes 102 with access to the fixed network104. The fixed network 104 can include, for example, a core local accessnetwork (LAN), and a plurality of servers and gateway routers to providenetwork nodes with access to other networks, such as other ad-hocnetworks, the public switched telephone network (PSTN) and the Internet.The network 100 further can include a plurality of fixed routers 107-1through 107-n (referred to generally as nodes 107, fixed routers 107 orwireless routers (WR)) for routing data packets between other nodes 102,106 or 107. It is noted that for purposes of this discussion, the nodesdiscussed above can be collectively referred to as “nodes 102, 106 and107”, or simply “nodes”.

As can be appreciated by one skilled in the art, the nodes 102, 106 and107 are capable of communicating with each other directly, or via one ormore other nodes 102, 106 or 107 operating as a router or routers forpackets being sent between nodes, as described in U.S. Pat. No.5,943,322 to Mayor, and in U.S. patent application Ser. Nos. 09/897,790,09/815,157 and 09/815,164, referenced above.

As shown in FIG. 2, each node 102, 106 and 107 includes a transceiver108 which is coupled to an antenna 110 and is capable of receiving andtransmitting signals, such as packetized signals, to and from the node102, 106 or 107, under the control of a controller 112. The packetizeddata signals can include, for example, voice, data or multimediainformation, and packetized control signals, including node updateinformation.

Each node 102, 106 and 107 further includes a memory 114, such as arandom access memory (RAM), that is capable of storing, among otherthings, routing information pertaining to itself and other nodes in thenetwork 100. The nodes 102, 106 and 107 take part in routing protocolwhich can be on demand or proactive and hence can send routing messageswhich can be Route Request, Route Reply, Route Error or RoutingAdvertisement whenever there is a change in the network topology or on aperiodic basis.

As further shown in FIG. 2, certain nodes, especially mobile nodes 102,can include a host 116 which may consist of any number of devices, suchas a notebook computer terminal, mobile telephone unit, mobile dataunit, or any other suitable device. Each node 102, 106 and 107 alsoincludes the appropriate hardware and software to perform InternetProtocol (IP) and Address Resolution Protocol (ARP), the purposes ofwhich can be readily appreciated by one skilled in the art. Theappropriate hardware and software to perform transmission controlprotocol (TCP) and user datagram protocol (UDP) may also be included.

As can be appreciated from the above, in the network 100, an IAP 106 isthe point of attachment of the wireless part and the wired Internet. Ifa Subscriber Device (SD) 102 is not in direct communication range withan IAP 106, the DS 102 depends upon other devices to reach the IAP 106.These devices can be other SDs 102 or Wireless Routers (WR) 107 whichare deployed specifically to provide coverage to these SDs.

In an ad-hoc network such as network 100, all nodes need to maintaincontinuous connectivity with the Access Point (or IAP) as most of thetraffic is to and from an IAP. This is true for both the fielddeployment and deployment in an office environment. Continuousconnectivity is also needed to tunnel the dynamic host configurationprotocol (DHCP) and address resolution protocol (ARP) IP broadcast tothe IAP. If an on-demand protocol, such as the Ad Hoc On-Demand DistanceVector Routing protocol (AODV), is used in an ad-hoc network, no routesare maintained proactively. Accordingly, the embodiments of the presentinvention described herein provide routes to IAPs that can be maintainedat all times with minimum overhead. This technique can also be used asan IAP association protocol. The technique has been named Proactive LAPLocator (PIL) protocol.

As can be appreciated by one skilled in the art, an on-demand routingprotocol (for example AODV) creates routes only when desired by thesource node. AODV is described in a publication by Charles E. Perkins,Elizabeth M. Belding-Royer, and Samir Das entitled “Ad Hoc On DemandDistance Vector (AODV) Routing”, RFC 3561, Jul. 2003, the entirecontents of which are incorporated herein by reference.

Hence, when a node requires a route to a destination, the node initiatesa route discovery process within the network 100. The commonly employedapproach, referred to as an Expanding Ring Search, can increase theaverage latency of route discovery, as multiple discovery attempts andtime-outs may be needed before a route to the target node is found, thehigh latency will require the source node to buffer the packets whichmay be difficult for memory constrained nodes in such a kind of networkresulting in packet loss. Furthermore, this elongated route discoveryprocess also increases the overhead as each discovery can lead to anetwork wide flood. Since most traffic in this kind of network flowsbetween IAP and other nodes, such as WRs and SDs, these floods can beavoided if the nodes proactively maintain routes to the IAP. This willalso avoid the latency and buffering of packets involved in findingroute to an LAP. AODV also assumes bidirectional links among nodes,which can lead to incorrect routes. The technique according to theembodiments of the invention described herein avoids unidirectionallinks while finding routes from nodes to the IAP, and vice-versa.

An example of the process will now be described with reference to FIG.3. In this example, it is assumed that all the nodes follow AODV routingprotocol with the modifications in accordance with the embodiments ofthe present invention described herein. All the nodes will periodicallybroadcast a packet that is referred to as IAP Advertisements (IA). Inplace of an IA packet, the nodes can also use a regular “Hello Message”which is generally sent by all nodes in such network to maintainconnectivity as described, for example, in a U.S. Provisional PatentApplication entitled “System and Method to Maximize Channel Utilizationin a Multi-Channel Wireless Communication Network”, Ser. No. 60/475,882,filed on Jun. 5, 2003, the entire contents of which being incorporatedherein by reference.

The contents of the IA packet can be any combination of the followingfields:

-   -   Type of node: This will let other nodes know about the type of        the device which can help them decide whether or not this node        should be used to route packets. This field can also help in        deciding the routing metrics as described in U.S. Provisional        Patent Application Ser. No. 60/476,237 referenced above.    -   Number of hops from the associated IAP.    -   Address of the node (IP or MAC address or both): This decision        is based on whether the network uses layer 2 routing or layer 3        routing or a combination of both.    -   Address of the associated IAP (IP or MAC address or both): This        decision is based on whether the network uses layer 2 routing or        layer 3 routing or a combination of both.    -   Routing Metrics to the associated IAP: This field helps in        deciding one route versus another as described, for example, in        U.S. Provisional Patent Application Ser. No. 60/476,237        referenced above, and in a U.S. Patent Application of Eric A.        Whitehill et al. entitled “EMBEDDED ROUTING ALGORITHMS UNDER THE        INTERNET PROTOCOL ROUTING LAYER OF A SOFTWARE ARCHITECTURE        PROTOCOL STACK”, Ser. No. 10/157,979, filed on May 31, 2002, the        entire contents of which are incorporated herein by reference.    -   Other Metrics (for example some metrics representing the load on        the IAP like number of active users associated with the IAP or        total bandwidth used by the users): this field can be used to do        load balancing across multiple IAPs and achieve quality of        service (QoS) goals across multiple IAPs.    -   QoS metrics: Used for QoS Routing.    -   Address of the node that is being used as next hop towards the        IAP: This field can be used to perform a “Split Horizon”        technique as done by classical distance vector protocols in the        Internet as described below.    -   Broadcast ID: a broadcast ID similar to that used in the AODV        route request (RREQ) process, which is helpful in detecting        duplicate packets and dropping those duplicates. This broadcast        ID may not be required if some sequence number is already a part        of MAC header to discard duplicate packets.    -   Power Level: This message can either be sent at some fixed power        that is known throughout the network, or power used should be        indicated in this field of the packet. This will help the node        receiving the packet to know the path loss between the        transmitter and itself.    -   TTL: The packet can also have a (time-to-live) TTL value set to        NETWORK_DIAMETER, which depends upon the size of the network 100        and the maximum number of hops possible between an IAP and a        node associated with that IAP. The TTL value can then be        decremented by each protocol interface layer (PIL) daemon as the        packet propagates throughout the network 100, which can control        the maximum number of hops possible between an IAP and any other        node in the network.    -   A node can also send similar information about some other        IAP/IAPs with which it is not associated. The information can        include all the metrics mentioned above.

Referring to FIG. 3, the process is started when an IAP broadcasts oneof these packets (step 1), and the nodes in the network 100 which areone hop away from the IAP receives it. On receiving such a message, thenode stores the relevant information from the message in a table andcompares that information against other entries if it has received thesame message from other neighboring nodes. It can also simply discardthe message if the next hop field in such a packet has its own address.This is done to avoid loops and is similar to the well-known “SplitHorizon” method used in conjunction with Distance Vector RoutingProtocol. As can be appreciated by one skilled in the art, Split Horizonis a well known method in wired networks to solve the“count-to-infinity” problem in distance vector protocol. Although thealgorithm is not perfect and the problem of “count-to-infinity” canstill exist if the number of nodes involved are more than 3, it is stilluseful. The algorithm selectively excludes a destination from anadvertisement to a neighbor, if the next hop to that destination is thatneighbor. Since the IA packet is a broadcast packet and sent in place ofa unicast packet to an individual neighbor, the split horizon methodcannot be used in its original form. Instead, the next hop used to reachthe destination (an IAP in the present example) can be advertised in thepacket. Hence, when a neighbor receiving the packet sees its address inthe next hop field, it ignores the advertisement to avoid the“count-to-infinity” problem. Thus, the split horizon method is used onthe receiver side in wireless network as opposed to being used on thetransmitter side as in a wired network.

These nodes will now make a routing decision based on number of factors,including but limiting to, the number of hops, routing metrics, loadbalancing metrics, QoS metrics, and so on. If a node decides to use thesender of this message as a next hop towards the IAP (the IAP itself inthe current example), then it sends a unicast Route Request (RREQ) forthe IAP address to the node (the LAP in this case) which forwarded theIA packet (step 2). Unicasting the RREQ will confirm that the link isnot a unidirectional link, but rather, a bidirectional link. If such aRoute Request fails, the node can blacklist the sender for some time andwait for IA messages from other nodes so that the node can hop throughthose other nodes to reach IAP. The nodes that are successful in theroute discovery process receive a Reply (RREP) back from the IAP (step3), and the IAP also creates a reverse route back to the node followingthe normal AODV routing protocol. This reverse route can be used to sendan IAP association/update message.

After obtaining the route, the nodes rebroadcast the IA message (step 4)after updating the relevant fields like increasing the hop count,decrementing the TTL, updating the different metrics etc. Nodesreceiving this IA will repeat the procedure by unicasting a RREQ (step5) to the forwarding node and receive a unicast Reply (RREP) from theforwarding node (step 6). It is noted that the G bit in this unicastRREQ will be set so that a Gratuitous RREP is also sent (step 7) to thedestination node (in this case, the IAP), so that the IAP also learns ofa route to the nodes. In this manner, knowledge of the IAP (which is thedefault route in network 100) is proactively flooded over the network100. As indicated in step 8, the nodes receiving the rebroadcast IA thenthemselves re-broadcast the IA (step 8).

In accordance with the AODV protocol, after a node receives a RREQ andresponds with a routing reply RREP, the node discards the RREQ. Ifintermediate nodes, such as other SDs or WRs, reply to everytransmission of a given RREQ, the destination (e.g., IAP) does notreceive any copies of the RREQ. In this situation, the destination doesnot learn of a route to the originating node. In an ad-hoc network, ifWRs always reply to the RREQs, the LAP will never learn about any routeto the SD. Currently, the AODV draft has a provision to let thedestination know about this route. Specifically, the AODV draft statesthat in order that the destination learn of routes to the originatingnode, the originating node SHOULD set the “gratuitous RREP” (‘G’) flagin the RREQ. If, in response to a RREQ with a ‘G’ flag set, anintermediate node returns a RREP, it must also unicast a gratuitous RREPto the destination node. Charles E. Perkins, Elizabeth M. Belding-Royer,and Samir Das. “Ad Hoc On Demand Distance Vector (AODV) Routing”,referenced above. This is the reason for enabling the G bit in the RREQpacket. However, in place of the G bit, a D bit (Destination only flag)can also be set so that noone other than destination (IAP in this case)replies to the message.

A summary of the Proactive IAP Locator Algorithms are as follows.

A Proactive IAP Locator Daemon of an IAP gets the IP or MAC address ofthe IAP, and builds an IAP Advertisement packet containing datapertaining to an IP/MAC address of the IAP. The associated LAP is set to0 as it is itself an IAP, the next hop towards the IAP is alsoinitialized to 0, the number of hops set to 0, and appropriate metricsare set. If the Network Broadcast ID is used it is initialized to 0, andif the power used for the message is not fixed, the quantized value ofthe power is placed in the power level fields, and the TTL is set to theNETWORK_DIAMETER value. The Proactive IAP Locator Daemon then broadcaststhe IAP Advertisement packet on all interfaces. The Proactive IAPLocator Daemon then repeats IAP Advertisement everyIAP_ADVERTISEMENT_INTERVAL seconds, which is a configurable parameter,while incrementing the Network Broadcast ID.

A Proactive IAP Locator Daemon of the WRs and SDs listens for an IAPAdvertisement packet containing data pertaining to an IP/MAC address ofan LAP, a number of hops from the forwarding node, different metrics,and so on, as mentioned above. If the node decides to use the sender asthe next hop towards the IAP, the Proactive IAP Locator Daemon issues aRREQ for IAP with a G bit or D bit set to the forwarding node. If anRREP is received, the Proactive IAP Locator Daemon sends anassociation/update message to the IAP, increments the Hops field,decrements the TTL, update other fields, and forwards to broadcastaddress on all interfaces. However, if no RREP is received, ProactiveIAP Locator Daemon waits for another IA packet and may elect toblacklist the sender for some time. It should be also noted here thatall nodes send the IA or Hello message on a periodic basis which can beconfigurable based on device type or other factors.

As can be appreciated by one skilled in the art, the technique accordingto the embodiments of the present invention described above avoids thehigh latency prone route discovery process for a common destination,such as an IAP, while minimizing overhead in the network 100. Thetechnique can be used as an IAP association protocol, and load balancingcan be achieved among different IAPs serving the network 100.

It is also noted that unicasting techniques similar to those discussedabove can be used to provide smooth handoff between mobile SD, such asthose being used in a moving vehicle. FIG. 4 shows a typical scenariowhere wireless routers WR1 and WR2 are deployed to provide coverage on ahighway. The circles represent the range of these WRs, which means thatany device, such as an SD, IAP or another WR will be able to communicatewith this WR if it is inside this circle. As it can be seen in thefigure, there is a small area where these two circles overlap. In thisarea, the SD is in position to communicate with both the wirelessrouters. It is advantageous for service providers to keep this area assmall as possible as it reduces the number of WRs that needs to bedeployed in a given area.

When a mobile SD is moving at highway speed, it rapidly crosses thecoverage area of the wireless routers WR1, and hence, passes by theoverlap area in a very small period of time. During this small timeperiod, the SD needs to change its routing table to reflect the factthat its best next hop to the infrastructure has changed from WR1 toWR2. A similar process is referred to as “handoff” in cellular networks.

In an ad-hoc such as network 100 described above, most of the trafficflows between an IAP and the SD, so in this small period of time, theIAP should also be informed about this change of point of attachment ofSD to the network 100. The embodiments of the invention described hereinprovide a system and method to achieve this and other objectives in afast and efficient manner. Specifically, the embodiments provide amethod which enables an SD to update its routing table to reflect thechange in its point of attachment to the network 100 (that is, itsaffiliation with a specific IAP) in a fast and efficient way withminimum loss of packets (called “Smooth Handoff”). An example of asmooth handoff technique is disclosed in U.S. patent application ofRobin U. Roberts and Charles R. Barker, Jr. entitled “A System andMethod for Performing Soft Handoff in a Wireless Data Network”, Ser. No.09/929,031, filed on Aug. 15, 2001, the entire contents of which isincorporated herein by reference. The embodiments also provide a methodfor other devices (in particular, LAP 106) to know about this change ina fast and efficient manner.

In a Distance Vector approach, devices such as SDs, IAPs and WRs learnabout the changes in routes through periodic routing advertisements.Thus, it takes substantial time (depending upon the periodic intervalbetween these advertisements) before an SD can know that it has movedaway from a WR and is close to another WR. This elapsed time is evengreater for an IAP which is typically several hops away from the placewhere routes have changed. This is illustrated in the following examplewhich will be discussed with reference to FIG. 5 in particular.

As shown in FIG. 5, a mobile SD is moving at a high speed on a highwaywhere coverage has been provided by wireless routers WR-3 through WR-6.In this example, the SD moves from left to right in FIG. 5 as shown,meaning that it is moving from the coverage area of WR-3 to WR-4. It isassumed that the time at which the mobile (SD) comes in communicationrange of WR-4 is t, and that the periodic interval between twoconsecutive Routing Advertisements is T (which is same for SDs and WRs).It is also assumed that there is a three way handshake mechanism whichconfirms that SD has indeed moved into the coverage range of WR-4 andthere exist a bi-directional link between them. This exchange can takeup to 3T periods. After this handshake, WR will start advertising theSD. The information will be propagated in following manner (in a worstcase scenario)

Time (t+3T): WR-4 is done confinning the presence of SD in its wirelessrange

Time (t+4T): WR-4 sends this information in its RA, WR-1 receives thisinformation.

Time (t+5T): WR-1 sends this information in its RA, IAP now knows aboutthis

Thus it takes (hops+2) times periodic time of RA to inform the IAP aboutthe change of route of SD, where hops=number of hops between SD and IAP

If T is large it will take a long time for the LAP to know about theroute change. Also, the time needed to perform the three way handshakecan also be so large that the SD actually moves out of the range of theWR involved in the handshake before the handshake process has beencompleted. One way to deal with this issue is to have a lower T.However, having a lower T substantially increases the overhead of thenetwork. The embodiments of the invention described herein thus providea technique in which this handshake operation can be performed in a moreefficient way so that the information can be propagated to the IAPwithout major overhead.

To perform this technique, all infrastructure devices, that is, all IAPsand WRs, are required to periodically broadcast a Neighbor Advertisementor Hello Message. This message can have similar fields as in the IApacket mentioned above. A three way handshake protocol can be used hereto confirm the bidirectional link.

All SDs actively monitor this Neighbor. Advertisement or Hello Messageand measure the signal strength of the signal. An SD can also do thethree way handshake to determine the received signal strength indicator(RSSI) and post detection signal quality (PDSQ) values on both ends. Assoon as the SD determines from the signal strength and/or these valuesthat it will loose connectivity with the old WR and should handoff, theSD unicasts an empty Routing Advertisement to the WR to which it wantsto handoff. It can also decide to handoff based on the routing metricsadvertised in the Routing Advertisement or calculated by the node. Inthe example shown in FIG. 5, if the SD is moving from the coverage areaof WR-3 to WR-4, the SD will send this unicast routing advertisement(RA) to WR4. This RA will be empty, that is, there will be no otherentries for other nodes but will have the header which will inform thereceiver node about this SD. The WR (i.e., WR-4) receiving this unicastRA will update its routing table in normal way, but since this was aunicast RA, it will also unicast one RA to the next hop towards the IAPwith which it is affiliated. In this example, upon receiving the RA fromSD, WR-4 will consult its routing table to find the best next hoptowards IAP, and will send a unicast RA to that node. This RA will justhave one entry about the SD. In this example, the best next hop to LAPis WR-1, which will follow the same procedure to direct the unicast RAtowards the LAP. Other message types can also be used in place ofexplicit RA, for example some geo packet can also be used to carry theinformation carried by routing advertisement. The information cansometimes be piggy backed to data packet as well.

As can be appreciated from the above, the SD can thus handoff in a verysmall time which does not depend upon the periodic RAs. Also, the IAPwill become aware of the movement of SD in real time rather than waitingto receive a periodic RA. Since this critical route update no longerdepends upon the broadcast RA, the periodic interval can be increasedwhich will result in substantially lower overhead. Hence, the embodimentachieves fast route convergence and low overhead because the frequencyof transmission of Routing Advertisements can be reduced.

Another smooth handoff technique, which is a modification to Ad HocOn-Demand Distance Vector Routing (AODV), will now be described withregard to FIG. 6.

As can be appreciated by one skilled in the art, AODV is a well knownon-demand routing protocol. Specifically, this type of routing protocolcreates routes only when desired by the source node. When a node, suchas an SD, WR or IAP, requires a route to a destination, such as anotherSD, WR or IAP, the node initiates a route discovery process within thenetwork 100. This process is completed once a route is found or afterall possible route permutations have been examined. Once a route hasbeen established, the established route is maintained by some form ofroute maintenance procedure until either the destination becomesinaccessible along every path from the source or until the route is nolonger desired.

Although the on demand approach reduces the routing overhead, it addslatency in obtaining a route since the routes are not computed beforethey are actually needed. Because of latency involved in finding routes,packets are needed to buffer the packets at the source. If the node ismemory constrained, packet losses can occur due to latency. Therefore,this approach generally does not facilitate smooth handoffs.

The following describes modifications to the standard AODV techniqueaccording to embodiments of the present invention, which facilitatesmooth handoffs in an ad-hoc network. These modifications also help toeliminate unidirectional links in the network 100, because AODV assumesbidirectional links.

FIG. 6 illustrates a portion of the network 100 in which WirelessRouters WR-1 and WR-2 are connected to IAP via wireless links. The LAPis connected to the core LAN as shown in FIG. 1. An SD is also shownwhich moves at a highway speed.

It can be assumed that most of the time, the SD will need a route to theIAP because the major applications performed by the SD will be Internetbrowsing, Voice Over Internet Protocol (VoIP) phone calls, and so on. Byusing the standard AODV routing protocol, much time is consumed beforethe route discovery process is completed. Furthermore, by the time theSD will receive a route reply, it is probable that the SD has moved todifferent position which may be outside of the range of a particular WRincluded in the route reply. The following technique according to anembodiment of the present invention avoids this drawback and helps inachieving smooth-handoffs of the SD from one WR or LAP to another.

In this example, it is assumed that all WRs and SDs are alwaysassociated with one LAP. IAPs and WRs periodically transmit a “HelloMessage” (or Neighbor Advertisement) which can have fields similar to IApacket described above. The fixed nodes can maintain the routes towardsthe IAP by sending a periodic RREQ as mentioned above. These nodes canalso maintain the route by just sending the RREQ the first time and thenby not expiring the route towards the IAP after getting the Route Reply.

To provide smooth handoff, the SDs will send an RREQ for theirassociated IAP at a periodic rate. This periodic interval is variabledepending upon various factors such as speed of the vehicle in which theSD is disposed (if the speed can be determined, for example, by notingthe average change of geo location, the activity of the SD, the rate ofchange of signal quality with the neighboring WRs, and an average numberof new neighbors added in the neighbor table). The Route Request packetsare generated with either the G bit or D bit set. Accordingly, thedestination (IAP) learns about the route without doing any RouteRequest. If a packet arrives at an LAP destined to some SD, the packetcan be delivered because the LAP knows the routes to all of the SDswhich are associated with that IAP.

It is also noted that the WRs should periodically broadcast neighboradvertisements in which they should include their IP address as well asthat of the IAP with which they are associated. The SD should cachethese advertisements and should periodically unicast the RREQ to theseWRs. If the link quality measurements are continuously made throughpassive listening of request-to-send/clear-to-send (RTS/CTS) or theseadvertisements, the SD can intelligently chose, for example, one tothree 1-3 of the WRs it is currently listening to (depending upon thenumber of back up routes desired) and unicast an RREQ to those WRs. Asstated, the number of WRs need not be within the range of one to three,but rather, can be any suitable number based on the number of back uproutes desired.

In place of sending periodic Route Request (RREQ) to several wirelessrouters, all the nodes in the network can simply use the Proactive IAPLocator protocol and achieve smooth handoff of mobile nodes. For this towork, the nodes should actively receive the IA/hello message/neighboradvertisement and should use such a combination of metrics that signifygood route in terms of throughput as well the signal strength betweenthe node and the next hop. For example, if the metrics involve thecombination of hops, node types, data rate, signal strength, batterypower, packet delivery ratio and so on, then as a mobile node moves awayfrom a WR (i.e., WR2) and moves closer to another WR (i.e., WR1) asshown in the FIG. 6, the cumulative routing metrics between the mobilenode and the IAP through WR2 will increase while one through WR1 willdecrease. As stated, it is assumed in this example that the mobile nodeis moving away from the coverage area of WR2 and is moving into thecoverage area of WR1. Also, it is assumed in this example that lowerrouting metrics represent a better route than higher routing metrics.Accordingly, the mobile node will automatically switch the routes andsend an RREQ to WR1 on determining that the routing metrics through WR1are better than through WR2, to thus achieve smooth handoffs.

As can be appreciated by one skilled in the art, this scheme has theadvantage of eliminating unidirectional links. As discussed above, theAODV protocol assumes bidirectional links and hence, a network 100operating in accordance with AODV creates a reverse route from a nodethat receives a RREQ to the node that made the RREQ as soon as thereceiving node receives the RREQ. It should be noted that if this linkis not bidirectional but rather, unidirectional, the AODV protocol canerroneously create incorrect routes. However, unicasting the RREQmessage confirms that the link is bidirectional since the hand-shake ofRTS-CTS is completed between the node that is sending the RREQ and thenode that is to receive the RREQ before the sending node sends the RREQ.Furthermore, the bidirectional link is verified because the node sendingthe RREQ receives an acknowledgement (ACK) back for the unicasted RREQfrom the node that receives the RREQ.

Accordingly, the technique described above provides for smooth handoffsof SDs and low latency in the network 100. Furthermore, each IAP in thenetwork 100 knows about the routes to all SDs associated with itself allthe time, and these routes between the LAP and its associated SDs areupdated in real time, which can help in reducing the route setup timefor phone calls originating from outside network, for example.Furthermore, overhead in the network 100 is minimized because theperiodic RREQ can be piggy backed with the data packets which arealready being sent from SDs, WRs and so on.

In addition, although all of the techniques described above are usedwith AODV in the examples given, these techniques can be used with otherrouting protocols and, in particular, on-demand type routing protocolssuch as dynamic source routing (DSR) or any other suitable protocol.

Although only a few exemplary embodiments of the present invention havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention.

1. A method for maintaining a route to an access point in a wirelesscommunication network, the method comprising: transmitting an accesspoint advertisement from the access point to at least one node of thenetwork in a broadcast range of the access point; transmitting a routerequest message from said at least one node to the access point uponreceipt of the access point advertisement with desired criteria by saidat least one node; transmitting a reply message from the access point tosaid at least one node upon receipt of the route request message by theaccess point; rebroadcasting the access point advertisement from said atleast one node to at least one other node of the network in a broadcastrange of said at least one node; transmitting a route request messagefrom said at least one other node to said at least one node upon receiptof the rebroadcast access point advertisement with desired criteria bysaid at least one other node; if a destination only flag of the routerequest message from said at least one other node is not set, thetransmitting a reply message from said at least one node to said atleast one other node, and a gratuitous reply message from said at leastone node to the access point, upon receipt of the route request messageby said at least one node; and if the destination only flag of the routerequest message is set, then forwarding the route request message fromsaid at least one node to the access point.
 2. A method as claimed inclaim 1, wherein: the network is a wireless ad-hoc peer-to-peer network,and said at least one node and said at least one other node communicatein the wireless ad-hoc peer-to-peer network.
 3. A method as claimed inclaim 1, wherein: the gratuitous reply message indicates to the accesspoint a route from the access point to the at least one other node.
 4. Amethod as claimed in claim 1, wherein: the access point advertisementtransmitting step transmits the access point advertisement from theaccess point to a plurality of said nodes of the network in a broadcastrange of the access point; the first route request message transmittingstep transmits a respective said route request message from each of saidplurality of nodes to the access point upon their respective receipt ofthe access point advertisement with desired criteria; the first replymessage transmitting step transmits a reply message from the accesspoint to each of said plurality of nodes upon receipt of the respectiveroute request messages by the access point; the rebroadcasting steprebroadcasts the access point advertisement from each of said pluralityof nodes to respective said other nodes of the network in a respectivebroadcast range of each of said plurality of nodes; the second routerequest message transmitting step transmits a respective said routerequest message from each respective said other node to the one of saidplurality of nodes from which it received the rebroadcast access pointadvertisement with desired criteria; the second reply messagetransmitting step transmits a respective said reply message from each ofsaid plurality of nodes that received a said route request message whosedestination only flag is not set to said respective other nodes withintheir broadcast range, and a respective said gratuitous reply messagefrom said each of said plurality of nodes to the access point, upontheir respective receipt of the respective route request messages whosedestination only flag is not set; and the forwarding step forwards thoseroute request messages whose destination only flag is set from saidrespective nodes at which they were received to the access point.
 5. Amethod as claimed in claim 4, wherein: each respective said gratuitousreply message indicates to the access point a respective route from theaccess point to a respective said other node.
 6. A method as claimed inclaim 1, wherein: said access point advertisement includes informationindicating a number of active nodes in the network associated with theaccess point.
 7. A wireless communication network, comprising: an accesspoint, adapted to provide access to a portion of the network; and aplurality of wireless nodes, adapted to communicate with each other andgain access to said portion of said network via the access point; theaccess point being further adapted to transmit an access pointadvertisement to said nodes which are in its broadcast range, the nodesbeing further adapted to transmit a respective route request message tothe access point upon receipt of the access point advertisement withdesired criteria, and the access point being further adapted to transmita respective reply message to the nodes upon receipt of the respectiveroute request messages; and each of the nodes being adapted torebroadcast the access point advertisement to other nodes of the networkin its respective broadcast range, to receive a respective route requestmessage from each of said other nodes that received the rebroadcastaccess point advertisement, and, upon receipt of the respective routerequest messages, to transmit a respective reply message to therespective other nodes and a respective gratuitous reply message to theaccess point for those of said respective route request messages whosedestination only flag is not set, and to instead forward to the accesspoint those of said respective route request messages whose destinationonly flag is set.
 8. A network as claimed in claim 7, wherein: thenetwork is a wireless ad-hoc peer-to-peer network, and said nodes andsaid other nodes are adapted to communicate in the wireless ad-hocpeer-to-peer network.
 9. A network as claimed in claim 7, wherein: saidaccess point advertisement includes information indicating a number ofactive nodes in the network associated with the access point.
 10. Amethod for performing handoff of a mobile subscriber device betweenfirst and second wireless routers in a communication network, the methodcomprising: when the subscriber device is moving from a coverage area ofthe first wireless router to a coverage area of the second wirelessrouter, unicast transmitting a routing advertisement, includinginformation identifying the subscriber device, from the subscriberdevice to the second wireless router; updating a routing table of thesecond wireless router in accordance with the information included inthe routing advertisement when the second wireless router receives therouting advertisement; and unicast transmitting another routingadvertisement, including information identifying the subscriber device,along a path of communication between the second wireless router and anaccess point via which the second wireless router gains access to aportion of the network, to inform the access point of the subscriberdevice.
 11. A method as claimed in claim 10, wherein: the network is awireless ad-hoc peer-to-peer network, and the first and second wirelessrouters and the subscriber device communicate in the wireless ad-hbcpeer-to-peer network.
 12. A method as claimed in claim 10, wherein: theunicast transmitting step unicast transmits said another routingadvertisement along said path which includes a plurality of wirelessrouters via which said another routing advertisement travels to reachthe access point.
 13. A wireless communication network, comprising: aplurality of wireless routers associated with an access point via whichthe wireless routers gain access to a portion of the network; and amobile subscriber device, adapted to communicate with any said wirelessrouter within its broadcast range; wherein when the subscriber device ismoving from a coverage area of a first wireless router to a coveragearea of a second wireless router, the subscriber device unicasttransmits a routing advertisement, including information identifying thesubscriber device, to the second wireless router, and when the secondwireless router receives the routing advertisement, the second wirelessrouter updates its routing table in accordance with the informationincluded in the routing advertisement, and unicast transmits anotherrouting advertisement, including information identifying the subscriberdevice, along a path of communication between the second wireless routerand the access point, to inform the access point of the subscriberdevice.
 14. A network as claimed in claim 13, wherein: the network is awireless ad-hoc peer-to-peer network, and the wireless routers, theaccess point and the subscriber device communicate in the wirelessad-hoc peer-to-peer network.
 15. A network as claimed in claim 13,wherein: the second wireless router unicast transmits said anotherrouting advertisement along said path which includes a plurality ofwireless routers via which said another routing advertisement travels toreach the access point.
 16. A method for performing handoff of a mobilesubscriber device between wireless routers in a communication network,the method comprising: controlling the mobile subscriber device totransmit at a periodic rate a routing request message for an associatedaccess point to the communication network; receiving the routing requestmessage at one of the wireless routers; and providing the routingrequest message from the wireless router at which it was received to theassociated access point to inform the associated access point of a routeto the mobile subscriber device.
 17. A method as claimed in claim 16,wherein: the periodic rate is based on a speed at which the mobilesubscriber device is moving.
 18. A method as claimed in claim 16,wherein: the network is a wireless ad-hoc peer-to-peer network, and thewireless routers, the access point and the subscriber device communicatein the wireless ad-hoc peer-to-peer network.
 19. A wirelesscommunication network, comprising: a plurality of wireless routersassociated with an access point via which the wireless routers gainaccess to a portion of the network; and a mobile subscriber device,adapted to communicate with any said wireless router within itsbroadcast range, to transmit at a periodic rate a routing requestmessage for an associated said access point to said any wireless router,which is adapted to provide the routing request message to theassociated access point to inform the associated access point of a routeto the mobile subscriber device.
 20. A wireless communication network asclaimed in claim 19, wherein: the periodic rate is based on a speed atwhich the mobile subscriber device is moving.
 21. A method as claimed inclaim 19, wherein: the network is a wireless ad-hoc peer-to-peernetwork, and the wireless routers, the access point and the subscriberdevice communicate in the wireless ad-hoc peer-to-peer network.
 22. Amethod for performing handoff of a mobile subscriber device betweenwireless routers in a communication network, the method comprising:controlling the mobile subscriber device to evaluate respective routingmetrics associated with the respective wireless routers as the mobilesubscriber device is moving; and controlling the mobile subscriberdevice to send a routing request message to the wireless router whoserespective said routing metrics meet a desired criteria.
 23. A method asclaimed in claim 22, wherein: the respective routing metrics includinginformation pertaining to factors which affect an ability of the mobilesubscriber device to communicate with its associated access point of thenetwork via a respective said wireless router.
 24. A method as claimedin claim 22, wherein: the network is a wireless ad-hoc peer-to-peernetwork, and the wireless routers, the access point and the subscriberdevice communicate in the wireless ad-hoc peer-to-peer network.